From the viewpoints of various advantages such as possibilities to make a size smaller and to make a film thinner with low voltage and low power consumption, liquid crystal display devices have been used widely for monitors of personal computers and mobile devices and televisions. In particular, in the liquid crystal displays used for televisions with a large screen expected to be viewed from various angles, demands fins view angle dependency become severe. Also, in recent years, performances requested for the liquid crystal displays used fire monitor become high. Accordingly, various modes are proposal so as to reduce view angle dependency by devising the arrangement state of liquid crystals in a liquid crystal cell. For example, according to the various studies of the liquid crystal display devices, an IPS (In-Plane Switching) mode, an OCB (Optically Compensatory Bend) mode, a VA (Vertically Aligned) mode, and the like are proposed.
As compared with other polymer films, since cellulose ester films are rich in moisture permeability, have high adhesive properties for the polarizer of the polarizing plate for which PVA with high hydrophilicity is generally used, and have high optical isotropic nature, the cellulose ester films are generally used as protective films for polarizing plates employed in various liquid crystal display devices. Accordingly, provision of the function of retardation films to the cellulose ester films enables to enhance visibility with a simple constitution without increasing the number of components.
Patent Document 1 discloses a retardation film provided with a desired retardation value by the addition of a retardation increasing agent to cellulose triacetate.
In the case where a retardation film is produced by stretching a cellulose ester film made for which a mixed fatty acid ester of cellulose with comparatively high processing adaptability such as cellulose acetate propionate is used as a raw material, Ro values can be made in a reversed-wavelength dispersibility.
On the other hand, cellulose esters are most inferior in moisture resistance. In particular, in the case where a total degree of substitution of acyl groups is lowered, for example, in the case of lowering it to 2.55 or less (hereafter, referred to as low-substituted cellulose ester), moisture vapor transmission and water content become high as compared with high-substituted cellulose ester. Accordingly, problems arise such that variations in dimension and retardation value during use become large.
As a means for solving these problems, it is known to improve the humidity stability of the retardation values of cellulose ester film to some extant by using a sugar ester compound as an additive of the cellulose ester film (for example, refer to Patent Document 2).
However, in a polarizing plate produced by applying saponification treatment to this retardation film and pasting the treated retardation film on a polarizer, new problems arise. For example, optical variations, such as the slippage of an optical axis (also referred to as “axis slippage) which causes the lowering of contrast and retardation variation, occur. Further, when the polarizing plate is placed under the environment of moisture and heat, inner dispersion occurs with an increase in inner haze. As a result, the retardation film is not in the state of satisfying the requirements as a retardation film.